One of the most impressive vascular beds is that of the counter- current heat exchanger in some pelagic fishes. Tunas are extraordinary fishes and one of the features that makes them extraordinary is their counter-current heat exchanger. This vascular device permits tuna to achieve body temperatures much warmer than ambient temperature--as much as 20 degrees C above ambient temperature in large bluefin tuna, whereas non-tunas rarely achieve excess body temperatures of more than 2 degrees C. Techniques are being used to: 1) describe in detail the anatomy of the vascular bed, and 2) experimentally demonstrate the effectiveness of this vascular bed as a thermal barrier in restrained skipjack tuna.